This disclosure relates to magnetorheological fluid compositions, and more particularly to high yield stress magnetorheological (MR) fluid compositions. The high yield stress fluid compositions generally include high aspect ratio magnetizable particles and low aspect ratio magnetizable particles disposed in a carrier fluid.
Fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field are referred to as Bingham magnetic fluids or magnetorheological fluids. Magnetorheological fluids generally include low aspect ratio magnetizable particles dispersed or suspended in a carrier fluid. In the presence of a magnetic field, the low aspect magnetizable particles become polarized and are thereby organized into chains of particles or particle fibrils within the carrier fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the fluid composition resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. When the flow of the fluid composition is restricted as a result of orientation of the particles into chains, the fluid composition is said to be in its “on state”. The force required to exceed the yield stress is referred to as the “yield strength”. In the absence of a magnetic field, the particles return to a disorganized or free state and the apparent viscosity or flow resistance of the fluid composition is then correspondingly reduced. The state occupied by the composition in the absence of a magnetic field is referred to as the “off-state”.
Commonly used magnetorheological fluids generally employ low aspect magnetizable particles that are symmetrical and have aspect ratios of 1 to 1.5. Examples of such particles are spherical particles, ellipsoids, cuboids, or the like. With reference now to FIG. 1, a typical prior art MR fluid device 10 contains an MR fluid composition 12 consisting of low aspect ratio particles 14 and a carrier fluid 16. As can be seen in FIG. 1, the low aspect ratio particles 14 form chains in the direction of an applied magnetic field 18. The formation of the chains promotes a selective increase in viscosity. Magnetorheological fluids employing the aforementioned particles are used in devices or systems for controlling vibration and/or noise. For example, magnetorheological fluids employing the aforementioned particles are utilized to provide controllable forces that can act upon a piston in linear devices such as dampers or mounts. In these applications, the magnetorheological fluid can be subjected to shear forces greater than or equal to about 70 kilo Pascals (kPa), at shear rates of about 1,000 to about 50,000 sec−1 causing extreme wear on the magnetizable particles. As a result of this wear, the magnetorheological fluid thickens substantially over time, leading to an increasing off-state viscosity. The increasing off-state viscosity leads to an increase in off-state force experienced by the piston or rotor. This increase in off-state force hampers the freedom of movement of the piston or rotor in certain off-state conditions.
In a magnetorheological device, it is often desirable to maximize the ratio of the on-state force to the off-state force in order to maximize the controllability offered by the device. Since the on-state force is dependent upon the magnitude of the applied magnetic field, the on-state force should remain constant at any given applied magnetic field. If the off-state force increases over time because the off-state viscosity is increasing but the on-state force remains constant, the on-state/off-state ratio will decrease. This decrease in the on-state/off-state ratio results in undesirable minimization of the controllability offered by the device. A more durable magnetorheological fluid that does not thicken over an extended period of time, preferably over the life of the device would be very useful. Moreover, it is desirable to minimize the costs of the MR fluid composition. Low aspect ratio magnetizable particles are relatively expensive to manufacture.
Accordingly, there is a need for improved MR fluid compositions suitable for high yield stress applications that are relatively less expensive.